Zinc Nitrate Hexahydrate Pseudobinary Eutectics for Near-Room-Temperature Thermal Energy Storage

Stoichiometric salt hydrates can be inexpensive and provide higher volumetric energy density relative to other near-room-temperature phase change materials (PCMs), but few salt hydrates exhibit congruent melting behavior between 0 and 30 °C. Eutectic salt hydrates offer a strategy to design bespoke PCMs with tailored application-specific eutectic melting temperatures. However, the general solidification behavior and stability of eutectic salt hydrate systems remain unclear, as metastable solidification in eutectic salt hydrates may introduce opportunities for phase segregation. Here, we present a new family of low-cost zinc-nitrate-hexahydrate-based eutectics: Zn(NO3)2·6(H2O)-NaNO3 (Teu = 32.7 ± 0.3 °C; ΔHeu = 151 ± 6 J·g–1), Zn(NO3)2·6(H2O)-KNO3 (Teu = 22.1 ± 0.3 °C; ΔHeu = 140 ± 6 J·g–1), Zn(NO3)2·6(H2O)-NH4NO3 (Teu = 11.2 ± 0.3 °C; ΔHeu = 137 ± 5 J·g–1). While the tendency to undercool varies greatly between different eutectics in the family, the geologic mineral talc has been identified as an active and stable phase that dramatically reduces undercooling in Zn(NO3)2·6(H2O) and all related eutectics. Zn(NO3)2·6(H2O) and its related eutectics have shown stability for over a hundred thermal cycles in mL scale volumes, suggesting that they are capable of serving as robust and stable media for near-room-temperature thermal energy storage applications in buildings.


SI 1. Variability in ZNH Melting Peaks Between Samples
SI Figure 1 DSC scans of two separate prepared samples of as-received 99.998% metals basis ZNH upon heating to evaluate the incipient melting peak observed on DSC.The red line indicated a single melting peak, whereas the black line indicated two melting peaks, suggesting excess water in the sample.
Separate samples of high-purity ZNH, all run at 1 °C•min -1 , were placed in the sample holder and backfilled with N 2. Some samples displayed unimodal behavior, and one showed bimodal despite extra cautions being taken to ensure purity of the ZNH.This indicates that there is somewhat stochastic behavior of a melt sample in terms of its melting shape despite all caution being taken to maintain the sample's purity.

SI 2. Solid-Liquid Transformation of ZNH
A thin film of ZNH was heated at 1 °C•min -1 , and the transformation associated with the minor DSC peak and the main melting peak (29 to 37 °C temperature range), initially observed with DSC were observed using transmission polarized optical microscopy (SI Figure 2).In 1 °C increments between photos, crystal boundaries are observed to begin melting during the initial peak below the main melting peak, at approximately 32 to 33 °C, indicating that this initial peak indicates incipient melting.No sign of solid-solid transformation is observed.This behavior is consistent with slightly excess Zn(NO 3 ) 2 present in the salt hydrate from the purification process, and initial melting occurring at the Zn(NO 3 ) 2 ⸱6(H 2 O)-Zn(NO 3 ) 2 ⸱4(H 2 O) eutectic point.To evaluate the use of the onset temperature as the rate-insensitive temperature, as expected for an invariant eutectic transformation, one pan of ZNH-NH 4 NO 3 was scanned using the Q2000 DSC at various ramp rates (1 to 10 °C•min -1 ).As heating ramp rates increased, the onset temperature deviated by < 0.5 °C, whereas the peak temperature deviated by nearly 1 °C (SI Table 1), with the faster ramp rates indicating a higher temperature melting peak, as expected due to internal temperature gradients within the sample volume.This indicates that the onset temperature is the more reliable value for a eutectic temperature.As-received talc and dried talc were scanned using powder-diffraction XRD to see if the drying process altered the structure of the talc (SI Figure 3).The drying process was applied to remove excess moisture which can lead to particle agglomeration and can change the water concentration of the salt hydrate.

SI
There was minimal difference between both patterns, indicating that heating the talc to 200 °C did not alter the structure.

Figure 2
Polarized light microscopy of ZNH upon heating to evaluate the incipient melting peak observed on DSC.SI 3. Evaluation of Onset and Peak Temperatures at Various Ramp Rates SI Figure 3 DSC curves highlighting different ramp rates with a black dashed line connecting onset temperatures and a purple dashed line connecting peak temperatures of the melting curve.The blue, red, and green lines represent the 1, 5, and 10 °C•min -1 ramp rates respectively.
SI 4. X-Ray Diffraction of As-Received vs Dried Talc SI Figure 4 X-ray diffraction pattern of as-received talc in blue and talc dried at 200 °C in red, with peaks indicated by tick marks on the bottom.

Figure 5
Cumulative distribution function (CDF) plots at various temperatures at various temperatures for ZNH, for (a) pure ZNH and (b) ZNH including 2 wt% talc.Colors represent isothermal temperature, as indicated.Dark blue x's represent the median values (τ 0.50 ).Solid lines represent the CDF of the collected data, and dotted lines represent the lower and upper bounds of the 95% confidence interval of the data collected at each temperature.SI Figure 6 Characteristic rates of ZNH (1/τ, dots).1/τ 0.25 , 1/τ 0.50 , 1/τ 0.75 are represented by the upwards triangle, x, and downwards triangle, respectively.Solid lines represent fit to 1/τ 0.50 , with upper and lower prediction bounds on the model.A red central line indicates samples with talc, and blue indicates neat.SI Figure 7 Cumulative distribution function (CDF) plots at various temperatures at various temperatures for ZNH-NaNO 3 , for (a) ZNH-NaNO 3 , and (b) ZNH-NaNO 3 , including 2 wt% talc.Colors represent isothermal temperature, as indicated.Dark blue x's represent the median values (τ 0.50 ).Solid lines represent the CDF of the collected data, and dotted lines represent the lower and upper bounds of the 95% confidence interval of the data collected at each temperature.SI Figure 8 Characteristic rates of ZNH-NaNO 3 (1/τ, dots).1/τ 0.25 , 1/τ 0.50 , 1/τ 0.75 are represented by the upwards triangle, x, and downwards triangle, respectively.Solid lines represent fit to 1/τ 0.50 , with upper and lower prediction bounds on the model.A red central line indicates samples with talc, and blue indicates neat.SI Figure 9 Cumulative distribution function (CDF) plots at various temperatures at various temperatures for ZNH-KNO 3 , for (a) ZNH-KNO 3 , and (b) ZNH-KNO 3 , including 2 wt% talc.Colors represent isothermal temperature, as indicated.Dark blue x's represent the median values (τ 0.50 ).Solid lines represent the CDF of the collected data, and dotted lines represent the lower and upper bounds of the 95% confidence interval of the data collected at each temperature.SI Figure 10 Characteristic rates of ZNH-KNO 3 (1/τ, dots).1/τ 0.25 , 1/τ 0.50 , 1/τ 0.75 are represented by the upwards triangle, x, and downwards triangle, respectively.Solid lines represent fit to 1/τ 0.50 , with upper and lower prediction bounds on the model.A red central line indicates samples with talc, and blue indicates neat.SI 6. Derived Slopes for Trendlines of Figure 10

Table 1 .
Onset and peak temperatures of ZNH-NH 4 NO 3 at different ramp rates